Frequency divider



April 18,

cup/25w 1939- D. A. BELL FREQUENCY DI VI DER Filed March 17, 1937 VOLTAGE VOLTAGE TIME TIME

I CURRENT -CURRENT INVENTOR DAVID A. BELL ATTORNEY Patented Apr. 18, 1939 UNITED STATES A 2,154,4sr

PATENT OFFICE FREQUENCYIDIVIDER Application March 17,

1937, Serial No. 131,325

In Great Britain April 29, 1936 2 Claims.

This invention relates to electrical frequency dividing circuit arrangements and has for its object to provide improved circuit arrangements capable of producing from a given input frequency a divided output frequency.

According to this invention an electrical frequency dividing circuit arrangement comprises two electron discharge devices, means for applying an input frequency to be divided in parallel to the input electrodes of said devices, a circuit resonant to the required divided output frequency included between the output electrodes of said devices, cross coupling means being connected between the output electrodes of each device and the input electrode of the other, but said cross coupling being insufficient to cause self-oscillation, and means for taking the divided frequency from the circuit included between the output electrodes, the operating potentials applied to the electrodes of said devices being such that current flows through each de-, vice only once during each complete period of the divided output frequency, there being no output in the absence of any input.

The invention is illustrated in and further explained in connection with the accompanying drawing in which Figure 1 shows, by way of example, one embodiment and Figs. 2 to 6 inclusive are explanatory graphical figures.

Referring to Fig. 1 the arrangement therein shown comprises two valves, shown as triodes I,

I 2, which have their cathodes 3, 4, connected together and to the negative terminal of a source 22 of anode potential. Each of the two triodes has its grid 5 or 6 connected through resistances I or 8 to a common input coupling circuit shown as a resistance-capacity coupling circuit 9, I0, though any other form of input coupling arrangement, such as a transformer, could be employed. The input wave whose frequency is to be divided is applied at the terminals II, the common grid circuit being completed to the negative terminal of the source 22 through a bias source I2 of such magnitude that in the absence of incoming alternating voltage at terminals II the anode currents of the valves are nearly zero. The triode anodes I3 and I4 are cross-connected to the grids 6, 5, as shown, through condensers I5, I6 which are sufficiently small to ensure that the system will not oscillate of its own accord. The two anodes I3, I4, are connected to opposite ends of a parallel tuned circuit I'I, I8, which is tuned to the desired sub-multiple (say one half) of the input frequency applied at I I, the center point I9 of the tuned circuit I'I, I8, ,being connected to the positive'terminal of the source 22 which is shunted by a suitable condenser 20. Output at the desired divided frequency (in the example above mentioned, one half that applied at terminals II) is taken off from the ends of the circuit I1, I8 to terminals 2|.

Consider an arrangement as shown in Fig. 1, and wherein the circuit I'I, I8, is tuned to one half the input frequency applied at II. Then the voltages applied to the two grids 5 and 6 will be as represented graphically in conventional manner in Figs. 2 and 3 respectively. From these figures it will be seen that in such a case the applied voltage is comprised of two components, one a component 7 of the input frequency at terminals II, and the other a component 172 of one-half this frequency. Since the valves have their input circuits fed in parallel from the terminals II, the component I will be the same in both cases but, since the valves are cross-connected to opposite ends of the circuit I'l, I8 (which is fed in opposition by the valves), the components f/2 for the two valves will be in phase opposition. Since the valves are so biased that in the absence of input alternating voltage at terminals I I, the anode currents are nearly zero, anode current will flow in any valve only at times when, as a result of input voltage at terminals II, the algebraic sum of the two component voltages on its grid is positive, the anode current in any valve being zero when the net voltage on its grid is negative. Fig. 4 shows in conventional manner the anode current curve which will result from the grid voltage components represented in Fig. 2, and Fig. 5 shows the anode current curve corresponding to Fig. 3. Since the outputs from the two valves are fed in phase opposition to the tuned circuit II, I8, the effective current to the said circuit will be as represented in conventional manner in Fig. 6, This is obviously a distorted wave of frequency f/2, and by virtue of the well known selective properties of a tuned circuit, the voltage developed across the tuned circuit Il, I8 will be considerably nearer the sinusoidal wave form than the current fed to it. It will have been noted that in Figs. 2 and 3 the components f/2 are shown as, sinusoidal, and in view of the action (just mentioned) of the tuned circuit I1, I8, this assumption is true to a reasonable degree of approximation; small distortions in the wave form applied to the grids 5, 6, are not of seriousimportance. The circuit will operate to give an approximately sinusoidal output of frequency .f/ 2 when fed with a frequency f, and although the wave form of the input voltage wave at terminals II will have some effect on the distortion of the output Wave form at terminals 2|, any reasonable input wave form of frequency i will result in an output of frequency f/ 2.

The factor of division in the above example is 2, but the invention may be employed to obtain division by any other even number, though as the number becomes greater the output becomes less. For division by the factor 2 the normal negative bias applied to the valve grids 5 and 6 is, as above stated, such as to reduce the normal anode current to zero or thereabouts. Where division by a larger number is required the negative bias should be so adjusted that the grids become positive only once in each cycle of the divided frequency.

It will be seen that the output circuit is balanced with respect to the input circuit both as regards coup-ling through the valves and coupling through the anode-grid cross-coupling means including the condensers l5. Hence the interaction between the input frequency and the divided output frequency circuits is reduced to a minimum. In other words, the output contains a very slight component of the input frequency, and, the impedance presented to the source of input frequency energy has a high value.

Although, regarded purely as a diagram, Fig. 1 resembles the well known multi-vibrator type of circuit, the invention is obviously fundamentally and inherently different from a multivibrator.

I claim:

1. A frequency divider comprising two electron discharge devices, each having a cathode and input and output electrodes and circuits connected thereto, a source of alternating current input energy coupled to the input circuits of said devices in parallel, a circuit resonant to a desired sub-multiple of the frequency of said source, said circuit being interposed between the output electrodes of said discharge devices, cross-coupling means between the output electrode of each de vice and the input electrode of the other, said cross-coupling means having an impedance value suflicient to prevent self-oscillation in said electron discharge devices, a source of operating potentials having a negative terminal connected to the input electrodes, an intermediate tap connected to the cathodes, and a positive terminal connected to the output electrodes of said devices, said positive terminal connection including a center tap on said resonant circuit, and means for maintaining the value of said source such that a substantial current flow through each device is permitted only once during each complete period of said sub-multiple frequency.

2. An electrical frequency dividing circuit comprising a pair of thermionic valves, each having a cathode, an anode and a control grid, crosscoupling condensers connected one between the anode-of one valve and the control grid of the other, and another between the anode of said other valve and the control grid of said one valve, said cross-coupling condensers being too small to cause self-oscillation, a parallel tuned circuit connected at one end to the anode of one valve and at the other to the anode of the other, means for applying an input frequency to be divided in parallel to the control grids of said valves, means for applying anode potential to said valves through a center point on said parallel tuned circuit, and means for taking the required divided output frequency from across said tuned circuit, said tuned circuit being resonant at said required divided output frequency. 7

DAVID ARTHUR BELL. 

